This invention relates to internal tube cutters and more particularly to internal tube cutters for internally cutting tubes of nuclear steam generators.
A typical nuclear steam generator comprises a vertically oriented shell, a plurality of U-shaped tubes disposed in the shell so as to form a tube bundle, a tube sheet for supporting the tubes at the ends opposite the U-like curvature, and a dividing plate that cooperates with the tube sheet forming a primary fluid inlet plenum at one end of the tube bundle and a primary fluid outlet plenum at the other end of the tube bundle. The primary fluid having been heated by circulation through the nuclear reactor core enters the steam generator through the primary fluid inlet plenum. From the primary fluid inlet plenum, the primary fluid flows upwardly through first openings in the U-tubes near the tube sheet which supports the tubes, through the U-tube curvature, downwardly through second openings in the U-tubes near the tube sheet, and into the primary fluid outlet plenum. At the same time, a secondary fluid, known as feedwater, is circulated around the U-tubes in heat transfer relationship therewith, thereby transferring heat from the primary fluid in the tubes to the secondary fluid surrounding the tubes causing a portion of the secondary fluid to be converted to steam. Since the primary fluid contains radio-active particles and is isolated from the secondary fluid by the U-tube walls and tube sheet, it is important that the U-tubes and tube sheet be maintained defect-free so that no breaks will occur in the U-tubes or in the welds between the U-tubes and the tube sheet, thus preventing contamination of the secondary fluid by the primary fluid.
Occasionally it is necessary to remove one or more of the heat transfer tubes from the tube bundle. In order to remove such a tube from the tube bundle it is first necessary to deactivate the steam generator and to cut the tube internally by entering the inlet plenum of the steam generator and extending a cutter into the tube. Once the cutter has been extended into the tube, the tube may be internally cut by the cutter and subsequently removed from the steam generator. However, some of the tubes chosen to be removed may be dented at intervals along the tube such that the cutter may not be able to traverse the tube. Therefore, it is desirable to have an internal tube cutter that is capable of being inserted into and traversing a constricted tube and yet capable of proper self-alignment within the tube such that the tube is properly cut.